118 HISTORICAL PALZAONTOLOGY. 
which are divided into several zones, capable of recognition by 
their fossils, and known by local names (Tentaculite Lime- 
stone, Water-lime, Lower Pentamerus Limestone, Delthyris 
Shaly Limestone, and Upper Pentamerus Limestone). As a 
whole, this series may be regarded as the equivalent of the 
Ludlow group of Britain, though it is difficult to establish any 
precise parallelism. ‘The summit of the Lower Helderberg 
group is constituted by a coarse-grained sandstone (the “ Oris- 
kany Sandstone”), replete with organic remains, which have 
to a large extent a Silurian facies. Opinions differ as to whether 
this sandstone is to be regarded as the highest bed of the Upper 
Silurian or the base of the Devonian. We thus see that in 
America, as in Britain, no other line than an artificial one can be 
drawn between the Upper Silurian and the overlying Devonian. 
As regards the “fe of the Upper Silurian period, we have, as 
before, a number of so-called ‘ Fucoids,” the true vegetable 
nature of which is in many instances beyond doubt. In addi- 
tion to these, however, we meet for the first time, in deposits 
of this age, with the remains of genuine land-plants, though 
our knowledge of these 1s still too scanty to enable us to con- 
struct any detailed picture of the terrestrial vegetation of the 
period. Some of these remains indicate the existence of the 
remarkable genus Lepidodendron—a genus which played a part 
of great importance in the forests of the Devonian and Carbon- 
iferous periods, and which may be regarded as a gigantic and 
extinct type of the Club-mosses (Lycopodiacee). _ Near the 
summit of the Ludlow formation in Britain there have also 
been found beds charged with numerous small globular bodies, 
which Dr Hooker has shown to be the seed-vessels or “ spor- 
angia” of Club-mosses. Principal Dawson further states that 
he has seen in the same formation fragments of wood with the 
structure of the singular Devonian Conifer known as /Profo- 
taxites. Lastly, the same distinguished observer has described 
from the Upper Silurian of North America the remains of the 
singular land-plants belonging to the genus Psz/ophyton, which 
will be referred to at greater length hereafter. 
The marine life of the Upper Silurian is in the main con- 
stituted by types of animals similar to those characterising the 
Lower Silurian, though for the most part belonging to different 
species. The Protozoans are represented principally by S¢ro- 
matopora and Ischadites, along with a number of undoubted 
sponges (such as Amphispongia, Astreospongia, Astylospongia, 
and Paleomanon). 
Amongst the Celenterates, we find the old group of Graf- 
tolites now verging on extinction. Individuals still remain 
